The goal of this proposal is to test two hypotheses regarding the use of radiolabeled peptide hormone receptor ligands for positron emission tomography (PET) imaging and targeted radiotherapy. We have demonstrated the effectiveness of 64Cu (T1/2 = 12.8 h) as a radionuclide for PET imaging and targeted radiotherapy in animal models. From the evaluation of a series of four somatostatin receptor (SSR) analogs, we have determined the optimal peptide sequence for high target uptake (Tyr3-octreotate (Y3-TATE)). In this proposal we will focus on how the structure of the bifunctional chelate (BFC) affects accumulation of activity in target and non-target organs. We will accomplish this by determining the metabolites of 64Cu-labeled BFC-Y3-TATE in vivo in non-target organs and in tumors, as well as the subcellular metabolism in xenografted tumors and tumor cells grown in culture. The first hypotheses we will address is that copper complexes that are the most difficult to reduce will demonstrate favorable clearance as Y3-TATE conjugates, compared to more easily reduced Cu(II) complexes. Our second hypothesis is that dissociation of 64Cu from the BFC-SSR analog may be advantageous in tumor cells, since 64Cu binding to nuclear proteins or DNA may increase its effectiveness of cell killing. Understanding the factors governing the retention of 64Cu in target and non-target tissues will aid us in the design of agents that have either more rapid non-target organ clearance or longer residence time in target tissues, or both. Depending on the clinical situation, there is a need for various imaging/therapy radionuclide pairs for diagnosing and treating cancer, including 86Y/90Y, 124I/131I, and 64Cu/64Cu (or 61Cu/64Cu). The second objective of this proposal is to use PET imaging to determine dosimetry and tumor response of DOTA-D-Tyr1-octreotate (DOTA-DY1-TATE) labeled with therapeutic amounts of iodine, yttrium and copper isotopes. Using a microPET scanner, which is a high resolution, small-bore PET scanner specifically designed for small animal imaging, we can first determine tumor and normal organ dosimetry, and then monitor the therapeutic response to radiolabeled DOTA-DY1-TATE. Accomplishing this goal will allow us to compare three therapeutic radionuclides labeled to the same agent with respect to efficacy and tumor absorbed dose, and also enable the non-invasive monitoring of non-subcutaneous tumors, such as liver metastases. The research proposed here will provide an understanding of the biological behavior of Cu(II) complexes, which is of importance to the field of diagnostic imaging and targeted radiotherapy with copper radionuclides. The research proposed here will also contribute to the development of new radionuclides for PET and targeted radiotherapy, and further the use of PET as a diagnostic modality prior to and during radiotherapy.